It has been conventional for a long time to produce pivot rods, such as the push rods that are utilized for operating the injection piston of a fuel injector or the cylinder valves of an engine of a construction having a tubular shaft into the ends of which pivot contact members constructed of a hardened material are plugged. An example of such a known push rod may be found in the DiMatteo, et al. U.S. Pat. No. 3,272,190. However, the high compressive loads imposed between, e.g., the ball and the socket components of pivot rods of such engine sub-system drive trains can result (within as little as 20,000 to 30,000 miles) in even hardened metallic surfaces of a ball and/or socket becoming worn to such an extent that undesirably large amounts of play occur which adversely impact upon the operation of the associated fuel injectors, valves, etc. Such wear is most common with either lower quality lubricating oils or with even good quality lubricating oil in which anti-wear additives have become depleted during the course of its use in an engine. Thus, when such wear occurs, it is necessary to perform major servicing of the engine and the vehicle equipped with the associated engine must be taken out of use for a day or more.
It has also been found that the use of ceramic components can produce a dramatic reduction in wear to such an extent that, even with a metal socket-ceramic ball combination, a life of as much as 500,000 miles can be expected before unacceptably large wear will have resulted (i.e., an increase of as much as 20 times the life of prior art metal-to-metal ball and socket joints). Thus, a definite advantage can be achieved if the pivot insert plugs for push rods and the like are made of a wear resisting ceramic material. On the other hand, a difficult problem exists in the design of ceramic tipped push rods concerning the attachment of the ceramic (i.e., silicon nitride, silicon carbide, zirconia, etc.) to the end of the tube.
When joining a metal plug to a metal tube, a "press fit" has normally been used as the means for attaching the plug inserts to the tube since that is the simplest and most economical method of attaching such parts. However, the problem that arises when doing this with ceramic end pieces is that the press fit induces a tensile "hoop" stress in the ceramic part which is directly proportional to the amount of "press" used to hold the ceramic end pieces. In metals, this is usually no great problem because of the ductility of the metal, but with ceramics too much tensile stress leads to possible fracturing of the ceramic piece. This fracture problem is compounded by the uncertainty in the "failure" criteria for such ceramic materials. While the amount of "press" can be controlled directly by strict tolerancing of the parts involved, this has been tried with the result that the "required" tolerances were not only uneconomically small, but were also unproducible with today's technology.
U.S. Pat. No. 4,508,067 to Fuhrmann discloses a tappet and a cam contact member therefor wherein a shaft-like solid tappet body made of, for example, cast iron, has an end socket in which a cam contact member made of a brittle, hard ceramic-based material is held by soldering or glueing. In order to reduce high-Hertz (contact) stresses, the contact surface is given a spherical surface having dimensioning that is determined in accordance with a special formula utilizing the maximum contact force expected between the expected between the cam contacting surface and cam, the Young's modulus of the material of the cam contacting surface, and the Poissons' ratio of the material of the cam contacting surface. Furthermore, the rear surface of the cam contacting member is flat and a concavity is provided between this rear surface and the bottom wall of the socket of the solid tappet body within which it is held so that the flat surface on the cam contact member opposite the cam engaging surface will always deflect toward the cavity during operation for reducing the contact stresses and wear associated therewith. However, numerous deficiencies exist in such a design. Firstly, it is hard to obtain a sufficient bond between a ceramic insert and a metallic body member by soldering or glueing. Furthermore, when soldering is used, adverse temperature effects are possible. Also, the precision machining associated with producing this type of contact member renders it considerably more expensive than a typical press fit mount, while the bending stresses associated with a design wherein a ceramic piece is "always" deflecting on contact could cause damage to the ceramic insert which is formed of an essentially brittle material.
A tappet with a wear resisting insert is also disclosed in Goloff, et al. U.S. Pat. No. 4,366,785. In this patent, the body of the tappet is a cylindrical piece formed, for example, of cast iron, steel, or the like, to which a disc-shaped wear resisting insert of a ceramic material is mounted within a complementary shaped recess in the end of the tappet body by way of an interference or press fit. By making the ceramic wear resisting insert of a disc shape with a flat, outer contact surface, and having this wear resisting insert fully received within the end recess of the tappet body, hoop stress problems are avoided, there being no tensile stress loading of the ceramic insert (tensile loading being the "Achilles heel" of ceramic materials, which are highly resistant to compressive loading). However, such a design has the disadvantage that it precludes the use of simple tube stock as a mounting shaft for a pivot insert, requiring instead a body member having a conforming recess with a bottom wall, which must be produced by the casting or machining. Moreover, the design of this patent is of limited applicability, since it cannot be used for attaching a wear resisting plug or insert in a manner which will result in the plug or insert being subjected to tensile hoop stresses, not merely compressive hoop stresses, e.g., where the insert projects axially beyond the end of the mounting shaft.